Time-dependent gas kinetics in tenuous planetary atmospheres: The cometary coma

The physics of steady and non-steady gas kinetic flow in the cometary coma has been explored using a new time-dependent kinetic model, which is based on Direct Simulation Monte Carlo (DSMC) methods and has been developed to study problems where the gas is not in local thermodynamic equilibrium so the validity of hydrodynamical approaches is questionable. The model is also applicable to a class of tenuous planetary atmosphere problems in addition to comets, e.g., Io, Pluto, and the upper atmospheres of the terrestrial planets and Titan. The results for a strong 2-D axisymmetric subsolar gas jet in a moderate comet (a few times 10(28) molecules sec(-1)) show that kinetic (non-fluid) effects become important in the flanks of the jet and on the night side in the immediate vicinity of the nucleus. Although the inner coma remains non-spherical, there is considerable flow (15%) into the night hemisphere (even in this dust-less model) and day-to-night winds with speeds in excess of 350 m sec(-1) are found near the terminator. Comparisons with new adaptive-grid hydrodynamic models show excellent agreement in the collisionally thick region of the jet, but more flow into the night side. A 1-D spherical steady-state model for six species (H2O, CO, OH, H-2, O, and H) at Giotto Halley flyby conditions is compared with the Giotto data, and contrasted with past hybrid fluid/ kinetic and recent iterative test particle kinetic models. Although the non-fluid nature of the 10(3)-10(5) km region of the coma is evident in all kinetic models and the qualitative behavior is similar, the ability of the DSMC model to include internal. (rotational) energy causes it to yield lower kinetic temperatures for H-2 and OH and enables it to determine explicitly the decreasing efficiency of LR rotational cooling in the outer coma due to non-LTE effects. The capability to model time-dependent kinetics is the strength of the DSMC method. A calculation has been performed for the 7-day periodic variation of Halley and shows that the variation in gas production rate produces a consequent temporal/spatial variation in outflow velocity. It is found that the amplitude of the gas production rate variation in March 1986, when taken directly from the typical constant-velocity analysis of photometric observations of cometary radicals, underestimates the true production from the nucleus by about 20-25%. Therefore, the amplitude of the periodic variation in March should be as large as that measured for April 1986 where the velocity effects are nil. (C) 1996 Academic Press, Inc.